The problems associated with non-uniform sample fluid flow at the entrance ports to photometric flow cells are well known and as listed in U.S. Pat. No. 4,374,620 include (1) instability in the optical base line between measurements of successive fluid fractions, (2) mixing between successive fractions, (3) formation of air bubbles in the flow cell passageway, and (4) formation of dead pockets at corners in the flow passageway which require relatively large volumes of fluid to totally sweep or flush each sample from the passageway.
In the '620 patent, the foregoing problems are addressed by the inclusion of a microporous annulus in the fluid inlet channel surrounding the entrance and/or exit port to and/or from the fluid passageway for receiving and passing fluid samples through the flow cell. Unfortunately, in practice, the economic manufacture of the microporous annulus has not been achieved nor has reliable fluid tight sealing of the marginal edges of the annulus in the inlet channel. For these reasons, the approach proposed by the '620 patent has not been commercially implemented.
Another approach to the foregoing problems is described and illustrated in an earlier U.S. Pat. No. 3,647,304. The '304 patent proposes a cylindrical insert having an inlet and an outlet and each defining a plurality of openings channelling to a generally cylindrical fluid chamber formed within the insert. The openings in the inlet are elongated and equidistantly disposed about the periphery of the inlet to provide a laminar fluid flow therethrough into the fluid chamber. While in theory it appears that the insert will reduce non-uniformities in fluid flow within the flow cell, the inlet flow of fluid through the plurality of equidistantly disposed radially extending openings is uneven. Moreover, the insert requires special and expensive manufacturing and introduces additional assembly expenses to the overall costs of the flow cell.
Accordingly, there is a continuing need for a low cost reliable solution to the problems associated with non-uniform entry and exit flow into and from fluid sample flow cells. The present invention satisfies that need.